I have seen some circuit breakers marked "Replacement Use Only - Not for CTL assemblies"
What is a CTL system? What do the letters "CTL" represent? What is the danger of using "Replacement
Use Only - Not for CTL assemblies" in a CTL assembly?

Answer

The history of CTL is somewhat clouded. You will hear many comments about what CTL
means (but most are wrong). If memory serves, CTL means "Circuit Total Limiting". "Circuit Total Limiting"
(CTL) was introduced when the words "A lighting and appliance branch-circuit panelboard shall be provided
with physical means to prevent the installation of more overcurrent devices than that number for which
the panelboard was designed, rated, and approved" was added to Section 384-15 in the 1965 NEC.

Here is a specific example:

Lighting and Appliance Panelboards (load centers) made since 1967 have been marked
"Class CTL Panelboard". Some of these CTL panelboards (load centers) will accept a breaker that is
other than full size (called tandems, half-size, wafers, etc.). These breakers have varying means
to limit the number of them that can be installed in the load center. One manufacturer has a hooked
foot on the tail end of the breaker that must mate up with a slot in the load center-mounting rail.
Others use a rejection feature in the breaker that must mount up with a "fork" in the busbar to be
installed. There are other arrangements as well.

When you hear somebody talk about a 30-40 load center (panelboard). It has 30 spaces for full size
breakers and 10 of those spaces have slots to that allow the installation of the "other than full
size" breakers. This 30-40 panelboard can contain 30 full size breakers and no tandem breakers, or
it can contain 20 full size breakers with 10 "other than full size" breakers for a total of 40 circuits.

These "other than full size breakers" cannot be installed in a panelboard that that does not contain
a "slot" in the mounting rail or a fork in the busbar, etc. Hence, the CTL panelboard has "physical
means to prevent the installation of more overcurrent devices than that number for which the panelboard
was designed, rated, and approved" in compliance with Section 384-15.

Keep in mind that many of these load center designs have been around since 1955. So what about panelboard
manufactured and installed prior to the design that incorporated the rejection feature? In order to
accommodate those designs, and have breakers that can still be installed, manufacturers have versions
of the breakers that do not have the rejection feature. These breakers are marked "Replacement Use
Only - Not for CTL assemblies."

What are the dangers on improperly using "Replacement Use Only - Not for CTL assemblies" in a CTL
assembly?

If you install "Replacement Breakers" (tandem breakers without a hooked foot) in a CTL panelboard,
the number of circuit breakers in the panelboard could exceed that number for which the panelboard
was designed, rated, and approved. The result could be an overheated panelboard or the breakers
might prematurely open (not stay in under load).

Installing "Replacement Breakers" in CTL panelboard is a violation of Section 110-3(b) and grounds
for the inspector to "red-tag" the installation.

The bottom line is... use the breakers that are intended to be installed in the panelboard
(load center). Your job will go much smoother.

Thanks to Jim Pauley of Square D for the short history of CTL circuit breakers.